The Cardiovascular System: Pulmonary and Systemic Circulation

Two Main Circuits of the Cardiovascular System

Pulmonary Circulation

Pulmonary circulation refers to the circulation of blood in which deoxygenated blood is pumped from the heart to the lungs and oxygenated blood is returned back to the heart. Pulmonary circulation only occurs between the heart and the lungs.

Systemic Circulation

Systemic circulation refers to the circulation of blood in which oxygenated blood is pumped from the heart to the body and deoxygenated blood is returned back to the heart. Systemic circulation occurs between the heart and the entire body.

Pericardial Sac

The pericardial sac is the outermost layer of the heart. It surrounds the heart and protects it.

Heart Valves

Atrioventricular (AV) Valves

The AV valves prevent backflow into the atria (closed for contraction).

Aortic and Pulmonary Valves

The aortic and pulmonary valves have three semilunar cusps and no chordae tendineae (open for contraction). They prevent backflow into the ventricles.

The two AV valves are the mitral valve (bicuspid valve) and the tricuspid valve, which are between the upper chambers (atria) and the lower chambers (ventricles). The two semilunar (SL) valves are the aortic valve and the pulmonary valve, which are in the arteries leaving the heart.

Coronary Arteries and Sinus

Coronary arteries remove the deoxygenated blood from the heart muscle, while coronary veins deliver oxygenated blood to the myocardium.

The coronary sinus drains blood from the heart tissue itself and drains into the right atrium.

Right and Left Sides of the Heart

Right Side

The right side of the heart handles deoxygenated blood. It is composed of the right atrium (upper right chamber of the heart), right ventricle (chamber underneath the right atrium), and tricuspid valve.

Left Side

The left side of the heart handles oxygenated blood. It is composed of the left atrium (left upper chamber of the heart), left ventricle (chamber underneath the left atrium), mitral/bicuspid valve (transfers blood from the left atrium to the left ventricle), and pulmonary artery.

Systole and Diastole

Systole (Contraction)

Systole is the phase of the heartbeat when the heart muscle contracts and pumps blood from the chambers into the arteries.

• Atrial systole: Blood under low pressure flows into the left and right atria from the pulmonary vein (left) and vena cava (right).
• Ventricular systole: Ventricles contract from the base upwards, increasing pressure and pushing blood up and out through the semilunar valves into the aorta on the left side and the pulmonary artery.

Diastole (Relaxation)

Diastole is the phase of the heartbeat when the heart muscle relaxes and allows the chambers to fill with blood.